Titanium dioxide is a semiconductor that has the property of exhibiting charge separation upon illumination with near-UV or ultraviolet (UV) radiation. Upon illumination, electrons are promoted from the valence band to the conduction band creating reactive electron-hole pairs that reduce oxygen to produce superoxide radicals and oxidize adsorbed water to produce reactive hydroxyl radicals, respectively. The holes have radical-cation character. All radicals have the ability to degrade the C—H bonds of organic compounds, mildew, mold, algae, grease, etc., thus imparting cleaning and self-cleaning properties to the oxide. As such, the incorporation of TiO2 into certain coatings will result in coatings that advantageously will retain self-cleaning properties.
The importance of clean coatings extends into corrosion aspects since organic and biological molecules attached to a surface can generate acids and other substances that, unless removed, can favor the onset of corrosion and/or biological corrosion of metal surfaces. Organic molecules attached to TiO2 may have beneficial effects on certain properties that relay upon charge separations, for example for solar energy conversion, but regular organic molecules are inefficient since they themselves contain C—H bonds, and thus are subject to degradation by radicals.
It should be noted that several publications provide compositions that include photocatalytic TiO2, such as WO2005/083014, WO 2006/030250, WO 2005/083013 and U.S. Pat. No. 8,475,581. However, inorganic TiO2 represents a fundamentally different composition of matter, with different physical and chemical properties, as well as photocatalytic properties compared with the organic fluoropigments of the current invention. This information is presented only in order to provide a better and complete understanding of the advantages of the present invention. It should not be viewed as a citation of prior art to the instant application.